1. Field of the Invention
This invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device with spacers to keep a gap between its substrates substantially constant.
2. Description of the Prior Art
A liquid crystal display device is light in weight, thin in thickness and low in power consumption so that it has been widely used in various applications. In such a liquid crystal display device, a twisted nematic (TN) type liquid crystal material is held between a pair of electrode substrates.
It is quite important to keep a gap between the electrode substrates substantially constant because, otherwise, undesired coloration occurs when the liquid crystal display device is in operation. In order to comply with such a requirement, ball-like spacers are scattered on one of the substrates in a predetermined density distribution. The substrate is then put together with another substrate. The gap is often referred to as a cell gap into which a liquid crystal material is put to complete a cell.
The ball-like spacers, however, can hardly achieve their uniform density distribution and the constant cell gap between the substrates. Further, some ball-like spacers pass light even on a light blocking mode of the liquid crystal display device. Thus, it is difficult to improve display contrast of the liquid crystal display device.
In order to solve the problems mentioned above, Japanese laid-open unexamined Patent Application Nos. 9-73093 and 9-73088 disclose column-like or square pillar spacers instead of the ball-like spacers.
The column-like or square pillar spacers are formed at predetermined places on one of the electrode substrates by using color filter materials at the same manufacturing process of color filter components. The spacer is configured to secure a uniform cell gap between the substrates and to improve display contrast.
The inventors of this patent application, however, still have found out the following problem during their research and development. Although a sufficient density of the column-like or square pillar spacers is required to maintain the load-resistance strength of a liquid crystal display device, gaseous vapor bubbles are generated between electrode substrates at lower operating temperatures. The generation of such bubbles depends on materials and density of the spacers. Change in volume of the liquid crystal material is not always adjustable with, but relatively different from, equivalent change in length of the spacers when operating or environmental temperatures fluctuate. Lower temperatures are particularly apt to cause bubbles. The bubbles affect display dignity of the liquid crystal display device.
One object of the present invention is to provide a high-dignity liquid crystal display device substantially unaffected by environmental temperature change in which spacers are disposed on a substrate.
According to one aspect of the present invention, a liquid crystal display device includes upper and lower substrates provided opposite to each other, a light modulation layer made of a liquid crystal material held between the substrates under a low pressure, a plurality of spacers formed on the upper and/or lower substrates to maintain a cell gap between the substrates and a display region made of a plurality of pixels. The liquid crystal display device is characterized in that change in the cell gap equivalent to constriction in volume of the liquid crystal material caused by temperature fluctuation from 20xc2x0 C. to xe2x88x9220xc2x0 C. is smaller than change in length of the spacers at the environmental temperature of xe2x88x9220xc2x0 C. on the condition that the pressure of 100 mN/mm2 is applied to the substrates between which the liquid crystal material is not held.
With that structure, the cell gaps can be adjusted in response to thermal constriction of the liquid crystal material at lower temperatures so that the generation of bubbles is prevented.
According to another aspect of the present invention, a liquid crystal display device also includes upper and lower substrate provided opposite to each other, a light modulation layer made of a liquid crystal material held between the substrates, a plurality of spacers formed on the upper and/or lower substrates to maintain a cell gap between the substrates and a display region including a plurality of pixels. This liquid crystal display device is characterized in that change X in length of the spacers at the environmental temperature of xe2x88x9220xc2x0 C. is more than 0.1 xcexcm while change Y at the environmental temperature of 20xc2x0 C. is less than 2X xcexcm on the condition that the pressure of 100 mN/mm2 is applied to the substrate between which the liquid crystal material is not held.
With this particular structure, the spacers sufficiently change in length even at the environmental temperature of xe2x88x9220xc2x0 C. to adjust the cell gaps in response to the thermal constriction of the liquid crystal so that the generation of bubbles are effectively prevented. Further, since the length change Y of the spacers at the environmental temperature of 20xc2x0 C. is controlled to be less than 2X xcexcm, the function of the spacers to maintain uniform cell gaps is not substantially affected. Thus, the generation of bubbles is further prevented while the uniform cell gaps are maintained in a wide temperature range.
The above-stated and other objects and advantages of the invention will become apparent from the following description when taken with the accompanying drawings. It will be understood, however, that the drawings are for purposes of illustration and not to be construed as defining the scope or limits of the invention, reference being had for the latter purpose to the claims appended hereto.